SAN FRANCISCO — Methane, which results from the decay of organic matter, wasn’t detected on Mars, suggesting the planet probably doesn’t support living microbes, according to new readings from the Curiosity rover.

The craft has identified no methane on its sensors, and the greatest amount of the gas that may exist in the atmosphere is about 1.3 parts per billion, about six times lower than previous estimates made by earth-bound telescopes and orbiting satellites, according to a paper released Thursday in the journal Science.

Earth has about 1,800 parts per billion of methane in the atmosphere, said Chris Webster, a study author and the director of the microdevices lab at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. The gas is mostly the result of the decay of organic matter, and a signal of biology. Higher readings of methane on Mars taken from Earth and orbiting satellites raised hopes that microbes might lie beneath the Martian surface, Webster said.

“There aren’t significant amounts of methane, so that paints a whole different picture,” Webster said. “We can say that there isn’t significant microbial activity.”

That doesn’t rule out the possibility that some microbes don’t produce methane, Webster said. The reading also doesn’t mean that life forms have never existed on Mars, he said.

“Curiosity established that it was a habitable world about a billion or less years ago, with water flowing and energy — the right ingredients for simple life forms,” Webster said.

Some previous readings found plumes of methane, which appear to have dispersed, with as much as 45 parts per billion, Webster said. Another report, from an orbiter, said that it detected 10 parts per billion, plus or minus 5. Some scientists have criticizing the methods used to make those readings.

“That’s in part because they’re very difficult observations,” Webster said. The sensor on Curiosity is more sensitive and doesn’t have to be read through Earth’s dense atmosphere, he said.

Though Curiosity is measuring in only one place, the atmosphere on Mars mixes and turns over every few months, Webster said. So if there were a plume somewhere, it would mix to form the background atmosphere being analyzed by Curiosity.